1. Field of the Invention
The present invention relates to an image forming apparatus and method for forming an image using image data to correct scan line curve that occurs during image forming.
2. Description of the Related Art
In recent years, a tandem system has often been employed to improve the image forming speed of color image forming apparatus (laser printers) utilizing an electrophotographic method. This system involves disposing plural types of coloring material (toner), an equal number of developers and a photosensitive drum, and forming a color image by sequentially transferring different colored images to a conveying belt or a recording sheet. With a color image forming apparatus employing this tandem system, a number of factors contribute to registration errors between the different colored images, since these images are formed with different image forming units corresponding to the respective colors. Various methods for correcting this have been proposed.
One contributing factor is thought to be nonuniformity between the lenses of the deflection scanner that deflects/scans the laser beam, displacement when the lenses are attached, or displacement when the deflection scanner is mounted to the image forming apparatus. When this happens, the scan lines of the laser beam slope or are curved. Since the extent of sloping or curving of the scan lines differs for each color, it appears in the form of registration errors.
As for methods of dealing with registration errors, in Japanese Patent Laid-Open No. 2002-116394 (Document 1), the amount of scan line curve in the deflection scanner is measured using an optical sensor during the assembly of the deflection scanner. Then, after adjusting the scan line curve by mechanically rotating the lenses based on the measurements, the lenses are secured with adhesive.
In Japanese Patent Laid-Open No. 2003-241131 (Document 2), the amount of scan line slope is measured using an optical sensor when mounting the deflection scanner to the color image forming apparatus, and the scan line slope is adjusted by mechanically inclining the deflection scanner according to the measured slope. After making the adjustment, the deflection scanner is mounted to the color image forming apparatus and fixed in place.
Further, Japanese Patent Laid-Open No. 2004-170755 (Document 3) discloses measuring the amount of scan line slope and curve in the deflection scanner using an optical sensor, correcting the bitmap image data to cancel out the slope and curve, and forming an image based on this corrected image data. The method of Document 3 does not require mechanical adjustment or adjustment during assembly, since registration errors are corrected by processing image data. Consequently, registration errors can be dealt with cost effectively, compared with the methods disclosed in Documents 1 and 2.
However, with the method disclosed in Document 3, line buffers for storing a plurality of scans worth of bitmap data are needed, since blend processing is performed to record the data of the adjacent line above or below the current line at different gradation levels. The number of line buffers depends on the width of the scan line curve. For example, if the scan line curve spans N lines of image data, enough line buffers are required to be able to store N lines of image data. Owing to variability between color image forming apparatuses, the value of N must accommodate a line count that exceeds the maximum value of this variability. When a large number of line buffers are thus secured, memory capacity is increased equivalently, and the circuitry for correcting bitmap image data is also increased, leading to a rise in costs.